Method of producing wafer and apparatus for producing wafer

ABSTRACT

A method of producing a wafer includes a peel-off layer forming step to form a peel-off layer in a hexagonal single-crystal ingot by applying a laser beam having a wavelength transmittable through the hexagonal single-crystal ingot while positioning a focal point of the laser beam in the hexagonal single-crystal ingot at a depth corresponding to the thickness of a wafer to be produced from an end face of the hexagonal single-crystal ingot, an ultrasonic wave generating step to generate ultrasonic waves from an ultrasonic wave generating unit positioned in facing relation to the wafer to be produced across a water layer interposed therebetween, thereby to break the peel-off layer, and a peel-off detecting step to detect when the wafer to be produced is peeled off the hexagonal single-crystal ingot by positioning an image capturing unit sideways of the wafer to be produced.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method of and an apparatus forproducing a wafer from a hexagonal single-crystal ingot.

Description of the Related Art

Devices such as integrated circuits (ICs), large scale integratedcircuits (LSIs), light emitting diodes (LEDs), etc. are formed on awafer of Si (silicon), Al₂O₃ (sapphire), or the like by depositing afunction layer on the face side of the wafer and demarcating the faceside into a plurality of areas by a grid of projected dicing lines.Power devices, LEDs, or the like are formed on a wafer of single-crystalSiC (silicon carbide) by depositing a function layer on the face side ofthe wafer and demarcating the face side into a plurality of areas by agrid of projected dicing lines. A wafer with devices formed thereon isdivided into individual device chips along projected dicing lines by acutting apparatus, a laser processing apparatus, or the like. Theproduced device chips will be used in electric appliances such as mobilephones, personal computers, and so on.

A wafer on which devices are to be formed is generally produced bycutting a thin slice off a cylindrical semiconductor ingot with a wiresaw. The wafer thus produced has its face and reverse sides polished toa mirror finish (see, for example, Japanese Patent Laid-open No.2000-94221). Such a customary process of slicing wafers off asemiconductor ingot with a wire saw and polishing the face and reversesides of the wafers is not economic because most (70% through 80%) ofthe semiconductor ingot is wasted. In particular, ingots of hexagonalsingle-crystal SiC are difficult to cut with a wire saw as they arehard, are poor in productivity as they take considerable time to cut,and pose problems in producing wafers efficiently as hexagonalsingle-crystal ingot are expensive to obtain.

The present applicant has proposed a technology in which a laser beamhaving a wavelength transmittable through hexagonal single-crystal SiCis applied to an ingot of hexagonal single-crystal SiC while positioningits focal point within the ingot, thereby forming a peel-off layer at aprojected severance plane in the ingot, and a wafer is peeled off fromthe ingot from the peel-off layer that serves as a severance initiatingpoint (see, for example, Japanese Patent Laid-open No. 2016-111143).

SUMMARY OF THE INVENTION

However, the proposed technology is problematic in that it is difficultand leads to poor production efficiency to peel wafers off a hexagonalsingle-crystal ingot, and that it is difficult to determine whether thepeeling of a wafer off a hexagonal single-crystal ingot is completed ornot.

It is therefore an object of the present invention to provide a methodof producing a wafer and an apparatus for producing a wafer which makeit easy to peel a wafer off a hexagonal single-crystal ingot from apeel-off layer therein that serves as a severance initiating point andwhich also make it easy to determine that the peeling of a wafer off ahexagonal single-crystal ingot is completed.

In accordance with an aspect of the present invention, there is provideda method of producing a wafer from a hexagonal single-crystal ingot,including: a peel-off layer forming step of forming a peel-off layer inthe hexagonal single-crystal ingot by applying a laser beam having awavelength transmittable through the hexagonal single-crystal ingotwhile positioning a focal point of the laser beam in the hexagonalsingle-crystal ingot at a depth corresponding to a thickness of a waferto be produced from an end face of the hexagonal single-crystal ingot;an ultrasonic wave generating step of generating ultrasonic waves froman ultrasonic wave generating unit positioned in facing relation to thewafer to be produced across a water layer interposed therebetween,thereby to break the peel-off layer; and a peel-off detecting step ofpositioning an image capturing unit sideways of the wafer to be producedand detecting when the wafer to be produced is peeled off the hexagonalsingle-crystal ingot.

Preferably, the hexagonal single-crystal ingot includes a hexagonalsingle-crystal SiC ingot having a c-axis and a c-plane perpendicular tothe c-axis, and the peel-off layer forming step of forming a peel-offlayer includes the peel-off layer that is made up of modified regionswhere SiC is separated into Si and C in the ingot and a succession ofcracks extending isotropically along the c-plane from the modifiedregions, by applying a laser beam having a wavelength transmittablethrough hexagonal single-crystal SiC to the hexagonal single-crystal SiCingot while a focal point of the laser beam is being positioned in thehexagonal single-crystal SiC ingot at a depth corresponding to thethickness of the wafer from an end face of the hexagonal single-crystalSiC ingot. Preferably, the hexagonal single-crystal ingot includes ahexagonal single-crystal SiC ingot having a c-axis inclined to a linenormal to an end face thereof and a c-plane, the c-plane and the endface forming an off-angle therebetween, and the peel-off layer formingstep of forming a peel-off layer includes the peel-off layer that isformed by continuously forming a modified region in the hexagonalsingle-crystal SiC ingot in a direction perpendicular to a direction inwhich the off-angle is formed, producing a succession of cracksextending isotropically along the c-plane from the modified region,indexing-feeding the hexagonal single-crystal SiC ingot and the focalpoint relatively to each other by a distance that is not larger than thewidth of the cracks in the direction in which the off-angle is formed,then continuously forming a modified region in the hexagonalsingle-crystal SiC ingot in the direction perpendicular to the directionin which the off-angle is formed, and producing a succession of cracksextending isotropically along the c-plane from the last-mentionedmodified region.

In accordance with another aspect of the present invention, there isprovided an apparatus for producing a wafer from a hexagonalsingle-crystal ingot having a peel-off layer formed therein by applyinga laser beam having a wavelength transmittable through the hexagonalsingle-crystal ingot while positioning a focal point of the laser beamin the hexagonal single-crystal ingot at a depth corresponding to athickness of a wafer to be produced from an end face of the hexagonalsingle-crystal ingot, an ultrasonic wave generating unit generatingultrasonic waves, the ultrasonic wave generating unit having an end facepositioned in facing relation to the wafer to be produced across a waterlayer interposed therebetween, an image capturing unit positionedsideways of the wafer to be produced, and a peel-off detecting meanscoupled to the image capturing unit, for detecting when the wafer to beproduced is peeled off the hexagonal single-crystal ingot based on achange in the gap between the wafer to be produced and the hexagonalsingle-crystal ingot.

With the method of producing a wafer according to the present invention,a wafer can easily be peeled off a hexagonal single-crystal ingot from apeel-off layer that serves as a severance initiating point, and thecompletion of the peeling of the wafer from the hexagonal single-crystalingot can easily be determined based on a change in the height of anupper surface of the wafer to be produced.

With the apparatus for producing a wafer according to the presentinvention, a wafer can easily be peeled off a hexagonal single-crystalingot from a peel-off layer that serves as a severance initiating point,and the completion of the peeling of the wafer from the hexagonalsingle-crystal ingot can easily be determined based on a change in theheight of an upper surface of the wafer to be produced.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an apparatus for producing a wafer,i.e., a wafer producing apparatus, according to an embodiment of thepresent invention;

FIG. 2 is a perspective view of parts of the wafer producing apparatusdepicted in FIG. 1, illustrating the manner in which an SiC ingot isbeing placed onto an ingot holding unit of the wafer producingapparatus;

FIG. 3A is a front elevational view of the SiC ingot;

FIG. 3B is a plan view of the SiC ingot;

FIG. 4A is a perspective view illustrating the manner in which apeel-off layer is being formed in the SiC ingot depicted in FIG. 3A;

FIG. 4B is a front elevational view illustrating the manner in which apeel-off layer is being formed in the SiC ingot depicted in FIG. 3A;

FIG. 5A is a plan view of the SiC ingot with a peel-off layer formedtherein;

FIG. 5B is a cross-sectional view taken along line B-B of FIG. 5A;

FIG. 6 is a front elevational view of parts of the wafer producingapparatus, illustrating the manner in which ultrasonic waves are beingapplied to the SiC ingot;

FIG. 7A is a schematic view of a binarized image of the SiC ingot beforeultrasonic waves are applied thereto;

FIG. 7B is a schematic view of a binarized image at the time the gapbetween a wafer to be produced and the SiC ingot exceeds a predeterminedvalue;

FIG. 8 is a front elevational view of parts of the wafer producingapparatus, illustrating the manner in which a wafer holding means isbeing held in intimate contact with a peeled-off wafer; and

FIG. 9 is a front elevational view of the parts of the wafer producingapparatus, illustrating the manner in which the peeled-off wafer isbeing held under suction by the wafer holding means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A method of and an apparatus for producing a wafer according to anembodiment of the present invention will be described in detail belowwith reference to the drawings. As illustrated in FIG. 1, an apparatus 2for producing a wafer, i.e., a wafer producing apparatus 2, according tothe present embodiment, includes an ingot holding unit 4 for holding ahexagonal single-crystal ingot (hereinafter simply referred to as“ingot”) thereon, an ultrasonic wave generating unit 6 having a lowerend face 6 a that faces a wafer to be produced from the ingot, forapplying ultrasonic waves to the ingot through a water layer interposedtherebetween, water supply means 8 for supplying water between the waferto be produced from the ingot and the ultrasonic wave generating unit 6to form the water layer, an image capturing unit 10 positioned sidewaysof the wafer to be produced, a peel-off detecting means 12 electricallyconnected to the image capturing unit 10, for detecting the peel-off ofthe wafer to be produced from the ingot based on a change in the gapbetween the wafer to be produced and the ingot, and wafer holding means14 holding a wafer peeled off the ingot.

The ingot holding unit 4 will be described in detail below withreference to FIGS. 1 and 2. The ingot holding unit 4 according to thepresent embodiment includes a cylindrical base 16, a cylindrical holdingtable 18 rotatably mounted on an upper surface of the base 16, and anelectric motor, not depicted, for rotating the holding table 18 about anaxis that extends vertically through the diametric center of the holdingtable 18. The ingot holding unit 4 holds the ingot thereon that issecured to an upper surface of the holding table 18 by an adhesive suchas an epoxy resin adhesive, for example. Alternatively, a porous suctionchuck, not depicted, connected to suction means, not depicted, may beplaced on the upper surface of the holding table 18, and the ingotholding unit 4 may hold the ingot on the porous suction chuck undersuction forces generated by the suction means and acting on an uppersurface of the porous suction chuck.

The wafer producing apparatus 2 further includes a Y-axis movingmechanism 20 for moving the ultrasonic wave generating unit 6, the watersupply means 8, and the wafer holding means 14 in a Y-axis directionindicated by the arrow Y in FIG. 1. The Y-axis moving mechanism 20 has aframe 22 in the shape of a rectangular parallelepiped having an elongaterectangular guide opening 22 a defined therein that extends in theY-axis direction, a first ball screw, not depicted, extending in theY-axis direction in the frame 22, a first movable arm 24 having aproximal end joined to the first ball screw and extending therefrom inan X-axis direction indicated by the arrow X in FIG. 1, a first electricmotor 26 connected to an end of the first ball screw, a second ballscrew, not depicted, extending in the Y-axis direction in the frame 22,a second movable arm 28 having a proximal end joined to the second ballscrew and extending therefrom in the X-axis direction, and a secondelectric motor 30 connected to an end of the second ball screw. When theY-axis moving mechanism 20 is in operation, the first ball screwconverts rotary motion of the first motor 26 into linear motion andtransmits the linear motion to the first movable arm 24, moving thefirst movable arm 24 in the Y-axis direction along the guide opening 22a. The second ball screw converts rotary motion of the second motor 30into linear motion and transmits the linear motion to the second movablearm 28, moving the second movable arm 28 in the Y-axis direction alongthe guide opening 22 a. The X-axis direction and the Y-axis directionextend perpendicularly to each other, and jointly define a substantiallyhorizontal XY plane.

As illustrated in FIG. 1, first lifting and lowering means 32 that is ofa cylindrical shape is connected to the lower surface of a distal end ofthe first movable arm 24. The ultrasonic wave generating unit 6 that isalso of a cylindrical shape is connected to the lower end of the firstlifting and lowering means 32. Therefore, when the first movable arm 24is moved in the Y-axis direction, the first lifting and lower means 32and the ultrasonic wave generating unit 6 are also moved in the Y-axisdirection. The first lifting and lowering means 32 may comprise anelectric cylinder assembly including a ball screw and an electric motor.The first lifting and lowering means 32 vertically moves the ultrasonicwave generating unit 6 and stops the ultrasonic wave generating unit 6in a desired vertical position, holding a circular lower end face 6 athereof in vertically facing relation to a wafer to be produced from theingot held on the holding table 18. The ultrasonic wave generating unit6 is made of piezoelectric ceramics or the like for generatingultrasonic waves.

As illustrated in FIG. 1, the water supply means 8 includes a tubularjoint port 34 mounted on the upper surface of the distal end of thefirst movable arm 24, a nozzle 36 vertically movably supported on thelower surface of the distal end of the first movable arm 24, and anozzle lifting and lowering mechanism, not depicted, for lifting andlowering the nozzle 36. When the first movable arm 24 is moved in theY-axis direction, the water supply means 8 is also moved in the Y-axisdirection. The joint port 34 is connected to a water supply, notdepicted, through a water supply hose, not depicted. The nozzle 36 isspaced from the ultrasonic wave generating unit 6 in the Y-axisdirection and extends downwardly from the lower surface of the distalend of the first movable arm 24. The nozzle 36 has a lower end portioninclined slightly downwardly and extends in the Y-axis direction towardthe ultrasonic wave generating unit 6. The nozzle 36 is of a hollowstructure and has an upper end held in fluid communication with thejoint port 34 and a lower end that is open as a nozzle outlet 36 a atthe distal end of the inclined lower end portion thereof. The nozzlelifting and lowering mechanism, which may comprise an electric cylinderassembly, for example, lifts and lowers the nozzle 36 and stops thenozzle 36 in a desired vertical position, positioning the nozzle outlet36 a between a wafer to be produced from the ingot on the holding table18 and the end face 6 a of the ultrasonic wave generating unit 6. Thewater supply means 8 thus constructed supplies water introduced from thewater supply into the joint port 34 through the nozzle outlet 36 a intoa space between a wafer to be produced from the ingot on the holdingtable 18 and the end face 6 a of the ultrasonic wave generating unit 6,thereby forming a water layer therebetween.

According to the present embodiment, as illustrated in FIG. 1, the imagecapturing unit 10 is disposed behind the nozzle 36 so that it will notbe sprinkled with water supplied from the nozzle outlet 36 a of thenozzle 36. The image capturing unit 10 is vertically movably supportedby a support bracket, not depicted. The image capturing unit 10 islifted and lowered by a lifting and lowering mechanism, not depicted,which includes an electric cylinder assembly, and is stopped in adesired vertical position. The image capturing unit 10 is positionedsideways of the wafer to be produced, and captures an image of apeel-off layer formed in the ingot and a gap between the wafer to beproduced and the ingot. The image capturing unit 10 may not be disposedbehind the nozzle 36 insofar as it is disposed in a position where itsimage capturing process is not obstructed by the water from the nozzleoutlet 36 a of the nozzle 36. The peel-off detecting means 12 that iselectrically connected to the image capturing unit 10 is supplied withan electric signal output from the image capturing unit 10. The peel-offdetecting means 12 includes a computer that includes a centralprocessing unit (CPU) that performs processing operations according to acontrol program, a read-only memory (ROM) that stores the controlprogram, etc., and a read/write random-access memory (RAM) that storesthe results of the processing operations. The peel-off detecting means12 detects when the wafer to be produced from the ingot is peeled offthe ingot based on a change in the gap between the wafer to be producedfrom the ingot and the ingot. Specifically, the peel-off detecting means12 binarizes the image captured of the ingot 50 by the image capturingunit 10, and detects the peel-off of the wafer from the ingot 50 basedon the binarized image at the time the gap between the wafer to beproduced and the ingot becomes equal to or larger than a predeterminedvalue.

As illustrated in FIG. 1, wafer holding means 14 is connected to thelower surface of a distal end of the second movable arm 28. Therefore,when the second movable arm 28 is moved in the Y-axis direction, thewafer holding means 14 is also moved in the Y-axis direction. The waferholding means 14 includes second lifting and lowering means 38 that isof a cylindrical shape extending downwardly from the lower surface of adistal end of the second movable arm 28 and a disk-shaped holder 40connected to the lower end of the second lifting and lowering means 38,for holding the wafer peeled off the ingot under suction. The secondlifting and lowering means 38, which may comprise an electric cylinderassembly, for example, lifts and lowers the holder 40 and stops theholder 40 in a desired vertical position, keeping a lower surfacethereof in contact with the wafer to be produced from the ingot. Aporous suction chuck, not depicted, that is connected to suction means,not depicted, is attached to a lower end face of the holder 40. Whilethe lower end face of the holder 40 is in contact with the upper surfaceof the wafer peeled off the ingot, the wafer holding means 14 holds thewafer on the porous suction chuck under suction forces generated by thesuction means and acting on the lower surface of the suction chuck.

FIGS. 3A and 3B illustrate an ingot 50 before a peel-off layer is formedtherein. The ingot 50 is made of hexagonal single-crystal SiC and has acylindrical shape as a whole. The ingot 50 has a circular first end face52, a circular second end face 54 opposite the first end face 52, aperipheral face 56 positioned between the first end face 52 and thesecond end face 54, a c-axis (<0001> direction) extending from the firstend face 52 to the second end face 54, and a c-plane ({0001} plane)perpendicular to the c-axis. The c-axis is inclined to a line 58 normalto the first end face 52, and the c-plane and the first end face 52 forman off-angle α (e.g., α=1, 3, or 6 degrees) therebetween. The directionin which the off-angle α is formed is indicated by the arrow A in FIGS.3A and 3B. The peripheral face 56 of the ingot 50 has a firstorientation flat 60 and a second orientation flat 62, each of arectangular shape, for indicating a crystal orientation. The firstorientation flat 60 lies parallel to the direction A in which theoff-angle α is formed, whereas the second orientation flat 62 liesperpendicularly to the direction A in which the off-angle α is formed.As depicted in FIGS. 3A and 3B, the length L2 of the second orientationflat 62 is smaller than the length L1 of the first orientation flat 60,as viewed from above (L2<L1). The ingot from which a wafer is peeled offby the wafer producing apparatus 2 after a peel-off layer has beenformed therein is not limited to the above ingot 50, but may be ahexagonal single-crystal SiC ingot where the c-axis is not inclined tothe line normal to the first end face and the off-angle between thec-plane and the first end face is 0 degree, i.e., the line normal to thefirst end face and the c-axis are aligned with each other, or ahexagonal single-crystal ingot made of a material other than hexagonalsingle-crystal SiC, such as GaN (gallium nitride) or the like.

A method of producing a wafer, i.e., a wafer producing method, accordingto the present embodiment will be described in detail below. Accordingto the present embodiment, a peel-off layer forming step is initiallycarried out to form a peel-off layer in the ingot 50 by applying a laserbeam having a wavelength transmittable through the ingot 50 to the ingot50 while positioning its focal point in the ingot 50 at a depthcorresponding to the thickness of a wafer to be produced from an endface of the ingot 50. The peel-off layer forming step can be performedusing a laser processing apparatus 64, which is partly illustrated inFIGS. 4A and 4B. The laser processing apparatus 64 includes a chucktable 66 for holding a workpiece, i.e., the ingot 50, thereon and a beamcondenser 68 for applying a pulsed laser beam LB to the workpiece on thechuck table 66. The chuck table 66, which is constructed to hold theworkpiece under suction on its upper surface, is rotatable about avertical axis by rotating means, not depicted, and is also linearlymovable in an x-axis direction by x-axis moving means, not depicted, anda y-axis direction by y-axis moving means, not depicted. The beamcondenser 68 includes a condensing lens, not depicted, for focusing andapplying the pulsed laser beam LB that is oscillated by a pulsed laserbeam oscillator, not depicted, of the laser processing apparatus 64, tothe workpiece. The x-axis direction is indicated by the arrow x in FIGS.4A and 4B, and the y-axis direction is indicated by the arrow y in FIG.4A. The x-axis direction is perpendicular to the y-axis direction, andthe x-axis direction and the y-axis direction jointly define asubstantially horizontal xy plane. The X- and Y-axis directions that areindicated by the arrows X, Y (capital letters) in FIG. 1 and the x- andy-axis directions that are indicated by the arrows x, y (lower-caseletters) in FIG. 4A may agree with or may be different from each other.

The peel-off layer forming step will be described in detail below withreference to FIGS. 4A and 4B. In the peel-off layer forming step, theingot 50 is held under suction on an upper surface of the chuck table 66while an upper end face, referred to as a first end face 52 in thepresent embodiment, of the ingot 50 is facing upwardly. The ingot 50 mayalternatively be secured to the chuck table 66 by an adhesive, e.g., anepoxy resin adhesive, interposed between a lower end face, referred toas a second end face 54 in the present embodiment, of the ingot 50 andthe upper surface of the chuck table 66. Then, an image capturing unit,not depicted, of the laser processing apparatus 64 captures an image ofthe ingot 50 from above the ingot 50. Based on the image of the ingot 50that is captured by the image capturing unit, the x-axis moving means,the y-axis moving means, and the rotating means of the laser processingapparatus 64 are controlled to move and rotate the chuck table 66 toadjust the direction of the ingot 50 to a predetermined direction and toadjust the relative position of the ingot 50 with respect to the beamcondenser 68 to a predetermined position in the xy plane. For adjustingthe direction of the ingot 50 to a predetermined direction, asillustrated in FIG. 4A, the rotating means is controlled to bring thesecond orientation flat 62 into alignment with the x-axis direction,thereby aligning a direction perpendicular to the direction A in whichthe off-angle α is formed with the x-axis direction and also aligningthe direction A in which the off-angle α is formed with the y-axisdirection.

Then, focal point position adjusting means, not depicted, of the laserprocessing apparatus 64 is controlled to lift or lower the beamcondenser 68 to position a focal point FP of the pulsed laser beam LB inthe ingot 50 at a depth of 300 μm, for example, corresponding to thethickness of a wafer to be produced from the first end face 52 of theingot 50, as illustrated in FIG. 4B. Then, a peel-off layer formingprocess is carried out. In the peel-off layer forming process, whilelinearly moving the chuck table 66 at a predetermined feed rate alongthe x-axis direction aligned with the direction perpendicular to thedirection A in which the off-angle α is formed, the beam condenser 68applies the pulsed laser beam LB whose wavelength is transmittablethrough the ingot 50, i.e., single-crystal SiC, to the ingot 50. Duringthe peel-off layer forming process, as illustrated in FIGS. 5A and 5B,the pulsed laser beam LB applied to the ingot 50 separates SiC in theingot 50 into Si (silicon) and C (carbon) and the pulsed laser beam LBsubsequently applied to the ingot 50 is absorbed by previously formed C,producing a modified region 70 where SiC is successively separated intoSi and C. The modified region 70 is continuously formed in the ingot 50along the direction perpendicular to the direction A in which theoff-angle α is formed. At the same time, a succession of cracks 72extending isotropically along the c-plane from the modified region 70are developed in the ingot 50. In the peel-off layer forming process,the beam condenser 68 rather than the chuck table 66 may be linearlymoved at a predetermined feed rate along the x-axis direction.

As illustrated in FIGS. 4A, 4B, 5A, and 5B, after the peel-off layerforming process, the laser processing apparatus 64 performs an indexingfeed process. In the indexing feed process, the y-axis moving meansindex-feeds the chuck table 66 relatively to the focal point FP alongthe y-axis direction aligned with the direction A in which the off-angleα is formed by a predetermined indexing feed distance Li, e.g., in therange from 250 to 400 μm, that is not larger than the width of thecracks 72. In the indexing feed process, the beam condenser 68 ratherthan the chuck table 66 may be linearly moved along the y-axisdirection. Then, the peel-off layer forming process and the indexingfeed process are alternately carried out to form a plurality of modifiedregions 70, each extending continuously along the directionperpendicular to the direction A in which the off-angle α is formed, atspaced intervals each equal to the indexing feed distance Li in thedirection A in which the off-angle α is formed, and also to form asuccession of cracks 72 extending isotropically along the c-plane fromthe modified regions 70, such that adjacent cracks 72 in the direction Ain which the off-angle α is formed vertically overlap each other. Inthis manner, a peel-off layer 74 made up of the modified regions 70 andthe cracks 72 is formed in the ingot 50 at the depth, which correspondsto the thickness of a wafer to be peeled off, from the first end face 52of the ingot 50. The peel-off layer 74 has a lower mechanical strengththan the remainder of the ingot 50, so that a wafer can be peeled offfrom the ingot 50 along the peel-off layer 74, as described later. Thepeel-off layer 74 can be formed in the peel-off layer forming step underthe following processing conditions:

Wavelength of the pulsed laser beam: 1064 nm

Repetitive frequency: 60 kHz

Average output power: 1.5 W

Pulse duration: 4 ns

Focal point diameter: 3 μm

Numerical aperture (NA) of the condensing lens: 0.65

Feed speed: 200 mm/s

The peel-off layer forming step is followed by an ultrasonic wavegenerating step in which the ultrasonic wave generating unit 6 ispositioned in facing relation to a wafer to be produced from the ingot50 across a water layer interposed therebetween and actuated to generateultrasonic waves that act through the water layer on the ingot 50 tobreak the peel-off layer 74. According to the present embodiment, in theultrasonic wave generating step, as illustrated in FIG. 2, the ingotholding unit 4 holds the ingot 50 thereon while the first end face 52that is closer to the peel-off layer 74 is facing upwardly. At thistime, the ingot 50 may be secured to the holding table 18 by an adhesivesuch as an epoxy resin adhesive, for example, interposed between thesecond end face 54 of the ingot 50 and the upper surface of the holdingtable 18, or by suction forces acting on the upper surface of theholding table 18. Then, the first motor 26 of the Y-axis movingmechanism 20 is energized to move the first movable arm 24 to positionthe lower end face 6 a of the ultrasonic wave generating unit 6 invertically facing relation to a wafer to be produced from the ingot 50,i.e., a portion of the ingot 50 from the first end face 52 to thepeel-off layer 74 according to the present embodiment, as illustrated inFIG. 1. Then, the first lifting and lowering means 32 lowers theultrasonic wave generating unit 6 until the distance between the firstend face 52 and the lower end face 6 a of the ultrasonic wave generatingunit 6 reaches a predetermined dimension, e.g., a value in the rangefrom approximately 2 to 3 mm. When the distance reaches thepredetermined value, the first lifting and lowering means 32 isinactivated. The nozzle lifting and lowering mechanism moves the nozzle36 to position the nozzle outlet 36 a of the nozzle 36 between the firstend face 52 and the lower end face 6 a of the ultrasonic wave generatingunit 6. Then, the holding table 18 is rotated by the electric motor.While the first motor 26 is moving the first movable arm 24 along theY-axis direction, water is supplied from the nozzle outlet 36 a of thenozzle 36 to a space between the first end face 52 and the lower endface 6 a of the ultrasonic wave generating unit 6, forming a water layerLW in the space, as illustrated in FIG. 6. The ultrasonic wavegenerating unit 6 is then energized to generate ultrasonic waves. Atthis time, the holding table 18 is rotated and the first movable arm 24is linearly moved along the Y-axis direction in order to pass theultrasonic wave generating unit 6 all over the first end face 52 of theingot 50. Therefore, the ultrasonic waves generated by the ultrasonicwave generating unit 6 are applied through the water layer LW to theingot 50 and hence to the peel-off layer 74 in its entirety. Theultrasonic waves applied to the peel-off layer 74 break the peel-offlayer 74, peeling off the portion of the ingot 50 from the first endface 52 to the peel-off layer 74 as a wafer 76 from the broken peel-offlayer 74 that serves as a severance initiating point.

In the ultrasonic wave generating step, the ultrasonic waves generatedby the ultrasonic wave generating unit 6 should preferably have afrequency close to the natural frequency of the ingot 50. By thussetting the frequency of the ultrasonic waves, the ultrasonic wavesapplied to the ingot 50 are able to peel off the wafer 76 from the ingot50 efficiently in a comparatively short period of time ranging fromapproximately one to three minutes even if the ultrasonic waves have acomparatively low output power level of approximately 200 W, forexample. The frequency close to the natural frequency of the ingot 50 isin a range from 0.8 to 1.2 times the natural frequency of the ingot 50.For example, if the natural frequency of the ingot 50 is 25 kHz, thenthe frequency close to the natural frequency of the ingot 50 ranges fromapproximately 20 to 30 kHz. Even if the ultrasonic waves have afrequency in excess of the frequency close to the natural frequency ofthe ingot 50, e.g., a frequency beyond 30 kHz, the ultrasonic wavesapplied to the ingot 50 are able to peel off the wafer 76 from the ingot50 efficiently in a comparatively short period of time providing theultrasonic waves have a comparatively high output power level rangingfrom approximately 400 to 500 W, for example.

In the ultrasonic wave generating step, the temperature of the watersupplied to the space between the first end face 52 of the ingot 50 andthe lower end face 6 a of the ultrasonic wave generating unit 6 shouldpreferably be set to a temperature at which the water layer LW isprevented from producing cavitation when the ultrasonic wave generatingunit 6 generates ultrasonic waves. Specifically, the temperature of thewater should preferably be set to a value ranging from 0° C. to 25° C.According to such a water temperature setting, the energy of thegenerated ultrasonic waves is not converted into cavitation, but iseffectively transmitted to the peel-off layer 74, efficiently breakingthe peel-off layer 74.

While the ultrasonic wave generating step is being carried out asdescribed above, a peel-off detecting step is carried out in which theimage capturing unit 10 is positioned sideways of the wafer 76 to beproduced, and the peeling of the wafer 76 from the ingot 50 is detected.If the peel-off detecting unit 12 detects the peeling of the wafer 76from the ingot 50, i.e., the peeling of the wafer 76 is completed, inthe peel-off detecting step, then the ultrasonic wave generating step isfinished. In the peel-off detecting step according to the presentembodiment, the image capturing unit 10 is first positioned sideways ofthe wafer 76 to be produced, and then captures an image of an upper endportion of the ingot 50. Then, the peel-off detecting means 12 binarizesthe image captured by the image capturing unit 10, and detects a gapbetween the wafer 76 to be produced and the ingot 50 based on thebinarized image. FIG. 7A schematically illustrates an image captured bythe image capturing unit 10 and binarized by the peel-off detectingmeans 12 before ultrasonic waves are applied to the ingot 50. In theimage illustrated in FIG. 7A, the thickness (e.g., approximately 100 μm)of the peel-off layer 74 is detected as a gap between the wafer 76 to beproduced and the ingot 50.

When ultrasonic waves are applied to the ingot 50, the peel-off layer 74is broken, and the gap between the wafer 76 to be produced and the ingot50 is widened. When the gap between the wafer 76 to be produced and theingot 50 is equal to or smaller than a predetermined value (e.g.,ranging from approximately 400 to 500 μm) that serves as a criterion asto whether the wafer 76 is peeled off the ingot 50 or not, the peel-offdetecting means 12 determines that the wafer 76 is not fully peeled offthe ingot 50, i.e., the wafer 76 to be produced is not wholly or partlypeeled off the ingot 50, and the ultrasonic wave generating stepcontinues. When the gap between the wafer 76 to be produced and theingot 50 becomes equal to or larger than the predetermined value, asillustrated in FIG. 7B, the peel-off detecting means 12 determines thatthe wafer 76 is peeled off the ingot 50. According to the presentembodiment, since the holding table 18 is rotating in the ultrasonicwave generating step, the peel-off detecting means 12 detects the gapbetween the wafer 76 to be produced and the ingot 50 along the entirecircumference of the ingot 50. The peel-off detecting means 12determines that the wafer 76 is peeled in its entirety off the ingot 50when the gap detected along the entire circumference of the ingot 50 isequal to or larger than the predetermined value. When the peel-off ofthe wafer 76 to be produced is detected, the ultrasonic wave generatingstep and the peel-off detecting step are finished.

After the ultrasonic wave generating step and the peel-off detectingstep have been carried out, the first electric motor 26 moves the firstmovable arm 24 to move the ultrasonic wave generating unit 6 and thenozzle 36 away from above the ingot 50, and the second electric motor 30moves the second movable arm 28 to position the wafer holding means 14directly above the ingot 50. Then, as illustrated in FIG. 8, the secondlifting and lowering means 38 lowers the holder 40 to bring the lowerend face of the holder 40 into contact with the first end face 52 of theingot 50. Then, the suction means connected to the suction shuck on theholder 40 is actuated to apply suction forces to the suction chuck onthe holder 40, which holds the peeled-off wafer 76 under suctionthereon. Then, as illustrated in FIG. 9, the second lifting and loweringmeans 38 lifts the holder 40, and the second electric motor 30 moves thesecond movable arm 28 to transport the peeled-off wafer 76 from theingot 50.

According to the present embodiment, as described above, the wafer 76can easily be peeled off the ingot 50 from the peel-off layer 74 thatserves as a severance initiating point, and the completion of thepeeling of the wafer 76 from the ingot 50 can easily be determined.According to the present embodiment, since the ultrasonic wavegenerating step is finished upon completion of the peeling of the wafer76 from the ingot 50, the period of time during which the ultrasonicwave generating step is carried out is not unduly increased, resultingin an increase in productivity of the wafer producing apparatus 2.According to the present embodiment, furthermore, the water layer LW isformed between the wafer 76 to be produced and the end face 6 a of theultrasonic wave generating unit 6 by supplying water to the spacetherebetween, and the ultrasonic waves are applied from the ultrasonicwave generating unit 6 to the ingot 50 through the water layer LW.Consequently, the wafer 76 can be peeled off the ingot 50 without usinga water tank in which wafer peeling would take place. The waferproducing apparatus 2 is economical as no time and no water is wasted tofill such a water tank.

In the peel-off layer forming step according to the present embodiment,the modified region 70 is continuously formed in the ingot 50 along thedirection perpendicular to the direction A in which the off-angle α isformed, and the ingot 50 is indexing-fed along the direction A in whichthe off-angle α is formed. However, the direction along which themodified region 70 is formed may not be the direction perpendicular tothe direction A in which the off-angle α is formed, and the direction inwhich the ingot 50 is indexing-fed may not be the direction A in whichthe off-angle α is formed. According to the present embodiment, thefirst lifting and lower means 32 for lifting and lowering the ultrasonicwave generating unit 6 and the nozzle lifting and lowering mechanism forlifting and lowering the nozzle 36 are separate from each other.However, the ultrasonic wave generating unit 6 and the nozzle 36 may belifted and lowered by a common mechanism mounted on the first movablearm 24, or the ultrasonic wave generating unit 6, the nozzle 36, and thewafer holding means 14 may be lifted and lowered by lifting and loweringthe frame 22 of the Y-axis moving mechanism 20.

The present invention is not limited to the details of the abovedescribed preferred embodiment. The scope of the invention is defined bythe appended claims and all changes and modifications as fall within theequivalence of the scope of the claims are therefore to be embraced bythe invention.

What is claimed is:
 1. A method of producing a wafer from a hexagonalsingle-crystal ingot, comprising the steps of: forming a peel-off layerin the hexagonal single-crystal ingot by applying a laser beam having awavelength transmittable through the hexagonal single-crystal ingotwhile positioning a focal point of the laser beam in the hexagonalsingle-crystal ingot at a depth corresponding to the thickness of awafer to be produced from an end face of the hexagonal single-crystalingot; generating ultrasonic waves from an ultrasonic wave generatingunit positioned in facing relation to the wafer to be produced across awater layer interposed therebetween, thereby to break the peel-offlayer; and positioning an image capturing unit sideways of the wafer tobe produced and detecting when the wafer to be produced is peeled offthe hexagonal single-crystal ingot.
 2. The method according to claim 1,wherein the hexagonal single-crystal ingot includes a hexagonalsingle-crystal SiC ingot having a c-axis and a c-plane perpendicular tothe c-axis; and the peel-off layer forming step of forming a peel-offlayer includes the peel-off layer that is made up of modified regionswhere SiC is separated into Si and C in the ingot and a succession ofcracks extending isotropically along the c-plane from the modifiedregions, by applying a laser beam having a wavelength transmittablethrough hexagonal single-crystal SiC to the hexagonal single-crystal SiCingot while a focal point of the laser beam is being positioned in thehexagonal single-crystal SiC ingot at a depth corresponding to athickness of the wafer from an end face of the hexagonal single-crystalSiC ingot.
 3. The method according to claim 2, wherein the hexagonalsingle-crystal ingot includes a hexagonal single-crystal SiC ingothaving a c-axis inclined to a line normal to an end face thereof and ac-plane, the c-plane and the end face forming an off-angle therebetween;and the peel-off layer forming step of forming a peel-off layer includesthe peel-off layer that is formed by continuously forming a modifiedregion in the hexagonal single-crystal SiC ingot in a directionperpendicular to a direction in which the off-angle is formed, producinga succession of cracks extending isotropically along the c-plane fromthe modified region, indexing-feeding the hexagonal single-crystal SiCingot and the focal point relatively to each other by a distance that isnot larger than the width of the cracks in the direction in which theoff-angle is formed, then continuously forming a modified region in thehexagonal single-crystal SiC ingot in the direction perpendicular to thedirection in which the off-angle is formed, and producing a successionof cracks extending isotropically along the c-plane from thelast-mentioned modified region.
 4. An apparatus for producing a waferfrom a hexagonal single-crystal ingot having a peel-off layer formedtherein by applying a laser beam having a wavelength transmittablethrough the hexagonal single-crystal ingot while positioning a focalpoint of the laser beam in the hexagonal single-crystal ingot at a depthcorresponding to the thickness of a wafer to be produced from an endface of the hexagonal single-crystal ingot, comprising: an ultrasonicwave generating unit generating ultrasonic waves, the ultrasonic wavegenerating unit having an end face positioned in facing relation to thewafer to be produced across a water layer interposed therebetween; animage capturing unit positioned sideways of the wafer to be produced;and peel-off detecting means coupled to the image capturing unit, fordetecting when the wafer to be produced is peeled off the hexagonalsingle-crystal ingot based on a change in the gap between the wafer tobe produced and the hexagonal single-crystal ingot.